In the past, we developed an in vitro system for studying replication and transcription using Sendai virus nucleocapsids extracted from infected cell. Recently, we have shown that treatment of Sendai virus nucleocapsids with S. aureus protease removes polypeptide P, but has little to no effect on polypeptide NP and L. To determine the function of P, we examined enzymatic activities in protease-treated nucleocapsids with the intention of correlating loss of P with loss of a specific function. We found no significant impairment in the capacity of P-deficient nucleocapsids to synthesize, methylate, cap, or polyadenylate viral mRNA in vitro. Studies with the RNA- mutant ts 105 demonstrated that (1) NP polypeptide was synthesized at the restrictive temperature, but was unable to assemble into mucleocapsids, and (2) replication was rapidly and completely inhibited, while transcription was not affected. Selection and analysis of antigenic mutants of Sendai virus with monoclonal antibodies demonstrated that the frequency of antigenic variation was about the same for Sendai virus, influenza virus, and VSV.